Producing better fuel cells with nanocrystals and graphene

Thin sheets of graphene oxide (red sheets) have natural, atomic-scale defects that allow hydrogen molecules to pass through while blocking larger molecules such as oxygen and water. By encapsulating nanoscale magnesium crystals (yellow) in these graphene oxide sheets, Berkeley Lab researchers were able to improve the performance of metal hydride fuel cells. Image: Jeong Yun Kim.

By surrounding hydrogen-absorbing magnesium oxide nanocrystals with sheets of atomically thin graphene oxide, researchers at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have managed to improve the performance of fuel cells that store hydrogen as metal hydrides.

The magnesium nanocrystals act as ‘sponges’ for hydrogen, offering a very compact and safe way to take in and store hydrogen. The nanocrystals also permit faster fueling, and reduce the overall ‘tank’ size. Meanwhile, the graphene oxide shields the nanocrystals from oxygen, moisture and contaminants, but allows the smaller hydrogen molecules to pass through via natural, atomic-scale defects. This filtering process overcomes common problems that degrade the performance of metal hydrides for hydrogen storage.

"Among metal hydride-based materials for hydrogen storage for fuel-cell vehicle applications, our materials have good performance in terms of capacity, reversibility, kinetics and stability," said Eun Seon Cho, a postdoctoral researcher at Berkeley Lab and lead author of a paper on this work in Nature Communications.

"This work suggests the possibility of practical hydrogen storage and use in the future," said Jeff Urban, a Berkeley Lab staff scientist and co-author. "I believe that these materials represent a generally applicable approach to stabilizing reactive materials while still harnessing their unique activity – concepts that could have wide-ranging applications for batteries, catalysis and energetic materials."

The research, conducted at Berkeley Lab's Molecular Foundry and Advanced Light Source, is part of a National Lab Consortium dubbed HyMARC (Hydrogen Materials--Advanced Research Consortium) that seeks safer and more cost-effective technologies for hydrogen storage. Urban is Berkeley Lab's lead scientist for this effort. According to Cho, a barrier to using metal hydrides for storage has been its relatively slow rate in capturing (absorption) and releasing (desorption) hydrogen during the cycling of fuel cells.

The tiny size of the graphene-encapsulated nanocrystals created at Berkeley Lab, which measure only 3–4nm across, means they can capture and release hydrogen much faster than conventional metal hydride materials, as it gives them a large surface area for reactions. The graphene oxide coating protects the magnesium from exposure to air, which would render the magnesium unusable by oxidizing it, she added.

Working at The Molecular Foundry, researchers found a simple, scalable and cost-effective ‘one pan’ technique to mix up the graphene sheets and magnesium oxide nanocrystals in the same batch. They later studied the coated nanocrystals' structure using X-rays at Berkeley Lab's Advanced Light Source. The X-ray studies showed how hydrogen gas pumped into the fuel cell reacted with the magnesium nanocrystals to form a more stable molecule called magnesium hydride, while oxygen was prevented from reaching the magnesium.

"It is stable in air, which is important," Cho said.

The next steps will focus on using different types of catalysts to further improve the fuel cell's ability to produce electricity and studying whether different types of material can also improve the fuel cell's overall capacity.

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